1. Field of the Invention
The present invention concerns an antenna with circular polarization, notably an elementary antenna for antenna arrays.
2. Description of the Prior Art
There are many circumstances in which it is desirable to have a circular polarization, notably in radar applications where it is known that circular polarization enables the elimination of the echoes produced by obstacles with isotropic reflection, especially rain echoes (caused by droplets of water that are in suspension in the clouds).
Indeed, the wave emitted in a given circular polarization, for example a right-hand circular polarization, will be phase-shifted by 180.degree. by reflection on the obstacle and will therefore be sent back with a reverse polarization, a left-hand circular polarization in this example. It will then be easy, at the receiver, to get rid of this reflection by means of a crossed polarization suppressor.
One of the aims of the invention is to propose an antenna with circular polarization such as this, notably to serve as a primary source (elementary antenna) in an array antenna, said antenna being capable of being supplied directly by a so-called symmetrical strip line.
A symmetrical strip line is constituted by a flat central conductor forming a coaxial cable core, sandwiched between two dielectric thicknesses (possibly air). These are themselves covered on their external surfaces by conductors located in front of the central conductor and supplied in parallel, hence conductors that are equipotential, forming peripheral ground conductors.
This symmetrical strip line technology is very common, especially in the array antennas, for it makes it easy to set up the complex distributors needed for the supply of the different primary sources of the array.
By contrast, one of the drawbacks of the symmetrical strip line technology lies in the fact that, until now, there has been no primary source with circular polarization directly extending the symmetrical strip supply line.
Indeed, the known primary sources with circular polarization (helical antennas, "candle" type antennas etc.) do not work in the same mode as the symmetrical strip line and therefore necessitate, in addition to the mechanical and electrical interfacing of the source with the symmetrical strip line, a change in excitation mode that is detrimental to optimal functioning of the source.
Besides, the radiating elements made up until now in symmetrical strip line technology do not provide any circular polarization and, therefore, in order to obtain a polarization mode such as this, it is necessary to add on polarizers to them, such as polarizers with dielectric strips, screws, wires, etc. with all the correlative matching losses and manufacturing difficulties.